The present invention relates to a safety device which reacts to the occurrence of a tear or break in a running paper web processed in rotary type roller offset printing machines, particularly those provided with printing mechanisms the damage to which must be prevented.
Due to a required relatively high paper web tension during the operation of roller rotary printing machines, an offset pressure on the rollers can easily cause tears in the web being printed on. A specific danger occurs in the region of the drier through which the web is guided over a relatively long distance in a contactless manner and where the web, due to a quick heating, is subjected to additional tensions.
Should a tear occur in a web during the operation of the rotary offset printing machine which runs with high speeds the portion of the web running until the machine is at standstill together with the web portion which is already positioned in the drier, is wound up on the cylinder of the last printing mechanism due to adhesion and decrease of web tension, whereby considerable damage can be done to bearings, cylinders and rubber cloth.
In order to avoid the aforementioned damage special devices have been proposed. One of such devices has been disclosed in DE-OS No. 32 15 473. For starting such a device a signal from web scanning elements is released upon the occurrence of a tear in the web by so-called web controllers. This signal is delivered to the electromagnet by which a detent mechanism is released by which the pivotable roller is pivoted relative to the stationary roller. The time between the release of said signal and the detection of the web is therefore relatively long.
In order to avoid this time consuming response, two rubber rollers driven with the speed approximately equal to the machine speed have been suggested in DE-PS No. 33 09 558. These rollers during a non-obstructive operation are resiliently adjusted relative to each other and are in connection with the web being printed on. These rubber rollers must be at the same time, pressure balanced. The disadvantage of this conventional device is its high cost, and, also the fact that both rollers must be moveable exactly in register with the cylinders of the printing mechanism to prevent smearing of the web. The provision of the roller rotary offset printing machines operated with high pressures with such devices is very expensive. High expense is also attributed to a required precise drive, expensive cylinder bearings, rubber tension devices and considerable wear of the rubber cloth of the rollers.